1. Field of the Invention
The present invention relates to an image forming method and to an image forming apparatus. More particularly, the present invention relates to an image forming method and to an image forming apparatus by use of an electroconductive polymer layer which takes in a dye and release the dye.
2. Description of the Related Art
Methods, which are currently utilized in a printer or the like for the purpose of transferring an image from an electric signal or optical signal to a recording medium such as paper, include, for example, dot-impacting, thermal transfer, thermal sublimation, ink jet and laser printing methods in electrostatic photography. These methods are roughly divided into three groups.
The methods, which are included in the first group, are those based on dot-impacting, thermal transfer and thermal sublimation. According to these methods, a sheet, which is in the form of an ink ribbon or a donor film and which contains a dispersed dye, is superposed on a paper or the like and the dye is transferred to the paper by the application of a mechanical impact or heat. Therefore, these methods always need expendable supplies and are associated with such disadvantages as high running costs due to difficulty in high-speed operation and poor energy efficiency. Further, the qualities of images are poor except for those obtained by the thermal sublimation method.
An ink jet method, which is included in the second group and is based on the mechanism that the ink is directly transferred to a paper from a head, is characterized by a low level of running costs, because no expendable supplies are necessary except for ink. However, the ink jet method is associated with a difficulty in high-speed operation, because it is difficult to electrically control all of the dots and to form a head corresponding to the width of a paper. Other disadvantages are that the minimal image unit is restricted by the size and interval of heads and that the printing speed decreases and the energy efficiency becomes poor with improving print quality.
An electrostatic photographic method such as a laser-printing method, which is included in the third group, is based on the mechanism that an image is formed via an intermediate transferring member. That is, according to an electrostatic photographic method, toner particles are caused to adhere to an electrostatic latent image created by laser spots and then the toner particles are transferred to a paper to forman image. This method is featured by a capability to form a relatively fine image and by a low level of running costs, because no expendable supplies are necessary except for toner. However, this method is associated with the problem that a high voltage is necessary for forming an electrostatic latent image and for adhering/transferring toner particles. Further, a large amount of electricity is consumed and, therefore, ozone and nitrogen oxides are generated. Furthermore, all of the above-mentioned printing methods generate noisy sounds when the printers are operated.
On the other hand, other conventional image forming methods, such as an ordinary printing method and a silver salt photographic method, provide images with high quality.
The ordinary printing method, which involves the formation of a printing plate and provides low running costs in the case that a number of identical images are formed, is not suitable for general uses. In the case of a silver salt photographic method, media, which are not reusable, such as photographic films and papers, need to be employed so that the running costs are high and a high-speed operation of the method is impossible.
As stated in the above, none of aforementioned methods, in which an image from an electric signal or optical signal is transferred to a recording medium such as paper, can provide a desirous method featured by a high-quality image, a relatively high speed, a low level of running costs, energy saving, resource saving and advantage both to environment and to users.
A conceivable means to solve the above-mentioned problems is the utilization of an image forming medium which can form and transfer or directly form an image distribution created by image forming elements such as toner particles or ink corresponding to an object image in such a manner that the image distribution is in the same scale (the same width) as that of an image receiving medium (such as a paper) onto which the image is transferred. Although this medium also functions as a temporary carrying member of the image forming elements, the intake or release (delivery) of the image forming elements needs to be performed with relatively low energy and a continuous gradation. Another required function is that the unit of the image forming element be minimized.
An image forming medium, which is considered to have the above-mentioned functions, is an electroconductive polymer layer represented, for example, by polypyrrole, polythiophene and polyaniline. It has been known that the three states, i.e., oxidized state, neutral state and reduced state, of this type of polymer layer can be controlled chemically, electrically or electrochemically so that doping or dedoping of a counter ion takes place. The details of these characteristics are shown, for example, "Electroconductive Polymers" by S. Yoshimura (Polymers Society of Japan), "Functions and Designs of Electroconductive Organic Films" by K. Yamashita and H. Kitani (Surface Science Society of Japan) and "Fundamentals and Applications of Electroconductive Polymers" by K. Yoshino (I.P.C.). In short, if an ion itself, which is doped into and dedoped from an electroconductive polymer layer is a sort of image forming element, the ion is expected to exhibit its ability as a temporary carrying member of the image forming elements that fulfill the aforementioned requirements.
One of the problems, however, is that the counter ions which are doped into and dedoped from an electroconductive polymer are those which cannot be expected to become an image forming element, such as usual metals or anions and cations of electrolytes having a low molecular weight. Another problem is that, if an electroconductive polymer is synthesized in the presence of a high molecular weight anion, for example, such an anion cannot be dedoped from the polymer.
According to H. Shinohara et al., described in J. Chem. Soc., Chem, Commun, pp. 87 (1986), the size of an ion, which can be reversibly doped/dedoped, is determined by the microstructure of an electroconductive layer and is controllable, for example, by the size of a counter ion present at the time when the electroconductive polymer is polymerized from a monomer. This report, however, deals with ions having a molecular weight up to about 100 and discloses that the doping/dedoping characteristic becomes insufficient as the molecular weight increases. H. Shinohara et al. also report, in Journal of Chemical Society of Japan, No. 3, pp. 465 (1986), glutamic acid as a relatively large molecule that exhibits a reversible doping/dedoping characteristic, but this molecular weight is still below 150. Meanwhile, a dye generally used, which can be expected to be an image forming element, has a molecular weight mostly in the range of 500 to 1,500. Heretofore, a substance having this level of molecular weight has never been thought to be capable of being doped/dedoped reversibly.
It has been known that an ion having a small molecular weight, as mentioned above, is subjected to doping into and dedoping from an electroconductive polymer layer in order to utilize a color change accompanied by the doping and dedoping. However, this kind of technique has been centered on such applications as protective coatings for battery or solar cell and electrochromic displays. Therefore, none of these techniques has been used to replace a conventional image forming method.
One of a few examples of a known technique, whereby an electroconductive polymer itself is used as a material related to a marking application, is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2-142,835 titled "A method for controlling wettability of a thin film surface of a polymer and an image forming method and an image forming material utilizing said method". According to this technique, a printing plate is prepared by electrically switching a difference in the wettability between an oxidized state and a neutral state of the electroconductive polymer layer. Therefore, none of image forming elements (such as ink) is held within the electroconductive polymer layer by way of doping. Further, an adhered amount or transferred amount of a dye such as ink cannot be controlled by this technique.
Based on the above-described background, the present applicant has already filed an application of an invention providing an image forming method and an image forming apparatus and a thin film of an electroconductive polymer usable therefor, which are featured by such advantages as a high-quality image, a relatively high speed, a low running cost, energy saving, resource saving and usefulness both to environmental preservation and to users (Japanese Patent Application No. 7-287,491 filed on Nov. 6, 1995, which is hereinafter simply referred to as "the prior application").
According to the invention of the prior application, a thin film of an electroconductive polymer is utilized for the purpose of providing an image forming medium, where an ionic dye is taken in among molecules of the thin film of the electroconductive polymer in at least one state of the electroconductive polymer capable of changing between at least two states of an oxidized state, a neutral state and a reduced state. This thin film of an electroconductive polymer is formed on an electroconductive substrate (the same as "electrode substrate" in the prior application) and is controlled in an oxidized state or in a reduced state corresponding to a target image so that a distribution of concentration in accordance with an intaken amount or a released amount of the ionic dye corresponding to the target image is formed on the thin film of the polymer. The distribution of concentration of the ionic dye thus formed is transferred to an image receiving medium onto which an image is transferred (the same as "recording medium" in the prior application) (for convenience, a medium consisting of an electroconductive substrate and the above-mentioned electroconductive polymer formed on the substrate is herein referred to as "an image forming medium").
Most generally, the transfer is conducted by the steps comprising bringing a thin film of an electroconductive polymer formed on an electroconductive substrate into close contact with an image receiving medium provided with an electroconductivity and applying an electric field between the thin film of the electroconductive polymer and the image receiving medium.
According to the method proposed by the prior application, the transfer of the dye in the form of a pattern was conducted in the following process.
The process comprised the steps of forming an image forming medium having a complex construction where a thin film of an electroconductive polymer was formed on each of a plurality of fine electroconductive substrates (fine electrodes) and then applying a voltage independently to each of the fine electrodes in conformity with an image pattern. Accordingly, the construction comprising a driving circuit, a driving means and an image forming medium was complex. In addition, an independent design was necessary for the driving circuit and the driving means.